The present invention relates to a new type of liquid crystal display where the input and output light beams do not follow the usual specular relationship.
Liquid crystal displays are usually manufactured with a structure as shown in FIG. 1. It comprises an input polarizer 1, a liquid crystal cell 2, an output polarizer 3 and a reflective diffuser 4. The liquid crystal cell is commonly made of two pieces of glass 5,6, alignment layers 7,8 conductive electrode films 9,10 and the liquid crystal material 11 which possesses a twisting alignment in conformance with the alignment layers 7 and
In this common reflective (or sometimes known as transflective) liquid crystal display, the light 12 enters the display from one direction at some azimuthal angle xcex8 relative to the surface normal 13 of the display. The corresponding polar angle of the incident light is xcfx86 relative to some x-axis on the surface of the display. Thus the angles specifying the light propagation direction is given by (xcex8,xcfx86). This light is scattered and reflected by the diffusive reflector and goes through the liquid crystal cell once more and is seen by the observer 14. This light intensity is strongest at the reflection angle (xcex8,xcfx86+xcfx80). This is called specular reflection or glare reflection. There is light observable at angles other than (xcex8,xcfx86+xcfx80) as shown because of scattering, but its intensity drops off rapidly as the angle deviates from xcfx86. The situation is depicted in FIG. 2. By the same scattering mechanism, at any viewing direction (xcex8,xcfx86+xcfx80), there is contribution of light incident from (xcex8,xcfx86), and light from incident angles near (xcex8,xcfx86). However, a majority of the light is from the (xcex8,xcfx86) direction.
In designing and optimizing such common liquid crystal displays, the alignment direction of the top and bottom glass plates and the placement of the input and output polarizers are crucial. If one takes the example of a 90xc2x0 twisted nematic liquid crystal display, the most common configuration is shown in FIG. 3. The input polarizer Pin and the input director nin are aligned at right angles. The output polarizer Pout is also perpendicular to the output liquid crystal director nout as shown. This is the so-called o-mode operation for the TN display. The light enters the liquid crystal display from the 12 O""clock direction 15 and the viewer looks at the display from the 6 O""clock direction 16. This is in contrast to the e-mode operation where Pin and nin are parallel, and Pout and nout are also parallel. The viewing angle polar plot for the o-mode TN display is shown in FIGS. 4 and 5. FIG. 4 is the polar plot for V=0 and FIG. 5 is the polar plot of transmittance for 2.5V. They show clearly the optimal viewing direction which is at the 6 O""clock position The darkest part of the polar plot in FIG. 5 indicates the light should exit the display at an azimuthal angle xcex8 of 30xc2x0 and a polar angle xcfx86 of 270xc2x0.
This optimization of the viewing angle of the liquid crystal display is well-known and has been discussed in the literature. For example, the books by Blinov et al (Electrooptic Effects in Liquid Crystal materials Springer-Verlag, 1994) and Bahadur (Liquid Crystals Applications and Uses, World Scientific, Singapore, 1990) have discussions on the viewing angle of liquid crystal displays. In these discussions, the light is assumed to traverse the liquid crystal cell at an oblique angle once. The viewing angle diagram plots the contrast of the display at the working voltages for light going through the liquid crystal cell at an angle of (xcex8,xcfx86) where xcex8 is the angle between the light beam and the surface normal of the liquid crystal cell (the azimuthal angle) and xcfx86 is the angle between the projection of the light bean on the liquid crystal cell surface and the reference x-axis (the polar angle). The input director of the liquid crystal is also measured referenced to this x-axis. For the case of the 90xc2x0 twist TN display, as shown in FIG. 3, the x-axis is usually taken to be at 45xc2x0 to the input director.
In the traditional optimization of the liquid crystal display, it is generally assumed that light enters at a certain angle. Many plots of the transmission-voltage curves have been shown in the literature for various combinations of the light viewing angle characterized by (xcfx86,xcex8). Implicit in such curves, with only one value of xcex8 specified, it is assumed that light enters and exits the cell at the same azimuthal angle. The possibility of light entering and exiting the liquid crystal cell at different azimuthal angles is never considered in the numerical and experimental optimization procedures. The present invention shows that for the case of nonspecular reflection, it is important to perform the simultaneous optimization of all important LCD parameters by considering light entering and exiting the LCD at different angles.
FIG. 6 shown the transmission-voltage curves for liquid crystal displays operating in the so-called second minimum. This second minimum corresponds to a retardation value, the product of the cell thickness and the birefringence of the liquid crystal (dxcex94n), of 1.075 xcexcm and a liquid crystal twist angle of 90xc2x0. Curve 17 is when the viewing angle and the light entrance angle are 0xc2x0 (normal to the cell). Curve 18 corresponds to light entering at xcex8=30xc2x0, xcfx86=90xc2x0 and the display is viewed at xcex8=30xc2x0, xcfx86=270xc2x0. This is the so-called 6 O""clock viewing condition. Curve 19 corresponds to conditions exactly opposite to curve 18, i.e. light entering at xcex8=30xc2x0, xcfx86=270xc2x0 and the display is viewed at xcex8=30xc2x0, xcfx86=90xc2x0. In the 6 O""clock position, the liquid crystal cell turns off at a lower voltage and the change in transmission as a function of voltage (the transmission-voltage or T-V curve 18 is sharper. This leads to a much better multiplexing capability for this display. FIGS. 7 and 8 are similar plots for the cases of 120xc2x0 and 180xc2x0 twist displays.
In this present invention, we recognize the fact that it is possible to manufacture LCDs where the input light angle and the output light angle are greatly different (non-specular reflection). Such a possibility of having non-specular light reflection was pointed out in U.S. Pat. No. 5,659,408 of M. Wenyon. One way of obtaining this situation is to use the so-called holographic reflector films (see, for example, M. Wenyon et al, xe2x80x9cWhite Holographic Reflector for LCDsxe2x80x9d, SID Symp. Dig. 1997). There are additionally many types of structured scattering surfaces that can achieve such nonspecular reflections. However, such prior LCDs do not optimize the reflection.
It is accordingly an object of the invention to seek to mitigate this disadvantage.
According to the invention there is provided a liquid crystal display, characterised by the incident light direction and the direction of light exiting the display after reflection being different directions which are non-specular.
Using the invention it is possible to provide that the incident and reflected light beams to be at different angles. The transmission-voltage curves should be calculated using different values of input and output angles.
A liquid crystal display embodying the invention, thus has all of its critical parameters simultaneously optimized allowing for the input light angle and the viewing angle to be different from each other, thus yielding retardation values of the display that are significantly different from conventional liquid crystal displays. The polarizer angles, the input/output directors and/or the liquid crystal cell retardation may thus be optimized for non-specular operation.
Another significant aspect of the present invention is the recognition of the fact that most of the nonspecular reflectors are monochromatic. That is, even with white light input, the reflected light will have a color, e.g. green. Hence the optimization of the nonspecular LCD does not have to take into account color dispersion effects. One can assume al monochromatic lights the input. Of course, this invention does not preclude the situation where the nonspecular reflector can be wide band or can reflect white light as well.
It is therefore possible using the invention to provide a set or sets of operating conditions for LCDs that are made with non-specular scattering reflectors. Such non-specular LCDs are classified into two broad categories: the image mode (i-mode) and the shadow mode (s-mode). In both the i-mode and the s-mode, the light comes into the LCD from the 12 O""clock direction with xcfx86=90xc2x0, and with xcex8 of typically about 30-45xc2x0. The reflected light exits the LCD at near normal incidence, which is the convenient direction for viewing an LCD.
In the i-mode, the polarizer directions and the input director of the liquid crystal cell are placed in the same manner as an ordinary liquid crystal display viewed at 6 O""clock. In this way, light enters the LCD from the 12 O""clock direction, and viewed at near normal. Using the 90xc2x0 TN LCD as an example, the resultant T-V curve will correspond to multiplying the T-V Curves 17 and 19 FIG. 6. As a comparison, in the conventional specular LCD with light incident from the 12 O""clock direction and viewed at the 6 O""clock direction, the overall T-V curve would correspond to multiplying curves 18 and 19.
In the s-mode, the entire polarizer-liquid crystal cell-analyzer assembly is rotated 180xc2x0 while the nonspecular reflector is not changed. In this way, light still enters from (30-45xc2x0, 90xc2x0) and is viewed at (0xc2x0,0xc2x0). However, the overall T-V curve should be represented by the product of curves 17 and 18 in FIG. 6. The most important observation is that the s-mode turns on much earlier at a much lower voltage than the i-mode device. Moreover, the overall transmission-voltage curve is much steeper in the case of the s-mode than the i-mode. Steeper T-V curve means that more data can be shown on the display with less cross talk. Both the nonspecular i-mode and s-mode are different from the specular LCD in terms of the T-V curve.
While FIG. 6 illustrates the idea of the present invention using the 90xc2x0 TN LCD, the same idea applies to all twist angles. For example, FIGS. 7 and 8 show the transmission-voltage curves for the case of 120xc2x0 twist and 180xc2x0 twist displays. It can be seen that there is a large difference between 6 O""clock light incidence and 12 O""clock light incidence as well. Thus for non-specular reflection displays, the arrangement of the polarizers and the directions of the viewing angle and light incident angle are critical in obtaining a good contrast display.
The fact that the taking into account of the angle of incidence in optimizing both the s-mode and i-mode display is illustrated by FIG. 9. In this Figure, we plot the transmittance of 120xc2x0 twist display as a function of the dxcex94n value of the liquid crystal. Here d stands for the thickness of the liquid crystal cell and xcex94n is the birefringence of the liquid crystal material. There are four curves with light incident angles ranging from 0xc2x0 to 60xc2x0. It can clearly be seen that the position of the first minimum (actually the first peak with 100% normalized transmittance) shifts to lower values as the light incidence angle increases. The difference between the 0xc2x0 case and 60xc2x0 case is as much as 50% decrease in dxcex94n.
The polarizer placement is also important in optimizing the i-mode and s-mode displays. Again, using the 120xc2x0 twist display as an example, FIG. 9 shows the transmittance as a function of dxcex94n for the polarizer arrangement shown in FIG. 10. It can be seen that the peak of the first minimum has shifted to a lower dxcex94n value as the light incident angle is increased to 60xc2x0. However, if the polarizer arrangements is changed to the one shown in FIG. 11, then the peak shifts to a larger value of dxcex94n as the light incident angle is increased, as shown in FIG. 12.
In FIG. 13, the need for optimization of the non-specular display is shown, taking into account both the angle of incidence and angle of reflection. In FIG. 13, we plot the change in transmittance as a function of dxcex9n for 0xc2x0 and 60xc2x0 angle of incidence. We also plot the product of the two curves since light will traverse the liquid crystal cell at these two directions. This represents the non-specular reflection case of light incident at 60xc2x0 degree to the display and viewed at near normal angle. It can be seen that the shifts in the peak of the first minimum is not as drastic, but nonetheless is still significant.